The present invention relates to a self-bonding insulated wire suitable for the production of deflecting yoke coil for use in television receiver, computer display and the like, and a self-bonding litz wire comprising it.
A self-bonding insulated wire has a bonding layer as the outermost layer. The self-bonding insulated wire is coiled around a mold and the outermost bonding layer is melted or swollen by electric heating or solvent treatment, whereby the adjacent wires can be adhered and solidified to form an integrated body. Therefore, self-supporting coils can be easily prepared therefrom. Like this, since the self-bonding insulated wire can improve the productivity of coils for use in electrical machinery and apparatus and can reduce the production cost, it has been popularly used in coils of household electric appliances, office automation machines, electric fixtures and accessories, deflecting yokes for CRT display, and the like.
In recent years, CRT display device is advanced in miniaturization, improvement of heat resistance, high voltage application and high frequency application. Therefore, the deflecting yoke coil which is an important part of the CRT display device, is required to be small in dimensional change at ordinary and high temperatures and in initial strain (which is a difference between the size of coil and the size of a mold for coiling and is evaluated by measuring the neck diameter of coil and the amount of torsion) observed when forming a coil by winding a wire around a coiling mold and subjecting the wound wire to melt-adhesion of the adjacent wires.
To meet these requirements, self-bonding insulated wires are required to have excellent heat distortion resistance and adhesive strength at both ordinary temperature and high temperature.
Also, recently, in order to reduce heat generation of coil per se by eddy-current loss and skin effect loss in deflecting yoke coil resulting from high resolution and high frequency application to CRT display, use of self-bonding litz wire wherein self-bonding insulated wires with small conductor size are stranded is becoming popular.
An epoxy resin (phenoxy resin) has been conventionally used as a melt-adhesive resin for forming the bonding layer of the self-bonding insulated wire, but recently is used a copolyamide which has well-balanced heat resistance and adhesive property.
The self-bonding insulated wires have been generally prepared by repeatedly applying and baking an insulating coating, e.g., polyester imide coating, polyester imide urethane coating or polyurethane coating, onto a conductor in plural times to give an insulated wire, applying onto the insulated wire a coating wherein a melt-adhesive resin such as a copolyamide resin is dissolved in a solvent such as cresol or an aromatic naphtha, by dies, placing the coated wire in a baking oven and evaporating the solvent to form a melt-bonding layer on the insulation layer. This method has the advantages that any resins can be used as a melt-adhesive coating so long as they are soluble in solvents and viscosity reduction required when applying the coating onto insulated wires is possible.
In melt-adhesive coatings conventionally used for the preparation of self-bonding insulated wires is used a large amount of a solvent over 85% by weight in order to reduce the viscosity to thereby achieve a good coating workability. However, the solvent per se is of course an unnecessary component for the bonding layer and is evaporated in a baking oven in a baking step during the preparation of self-bonding insulated wires. The use of a large amount of a solvent is also disadvantageous in that the concentration of the effective film-forming component for forming the bonding layer becomes low and, therefore, the thickness of the film formed by one coating-baking operation is thin. Thus, the coating-baking operation must be repeated multiple times to form the bonding layer having a desired thickness. It is very uneconomical.
Further, in recent years, resources saving and energy saving are strongly demanded, and effective utilization of resources is important. The effective utilization of resources would also bring a favorable result to environmental pollution problem.
In light of the above, if the amount of a solvent in the melt-adhesive coatings can be decreased, it would be very advantageous in economy and productivity and would be effective for resources saving and environmental pollution prevention.
Phenolic organic solvents such as cresol are generally used in conventional melt-adhesive coatings. The conventional melt-adhesive coatings also have the problem that since the amount of such phenolic solvents is large, evaporation of the solvents in the baking step is apt to become insufficient, so a trace amount of the solvents remains in the bonding layer (the amount of residual solvents being at least about 1.0% by weight based on the weight of the bonding layer of the self-bonding insulated wire). The residual solvents cause the problem that they evaporate during electrical heating in the coiling working. Since the phenolic organic solvents such as cresol are harmful from the viewpoints of odor and environment, it is desirable to decrease the residual solvents in the bonding layer as much as possible.
In order to solve the residual solvent problem, JP-A-8-17251 proposes to use benzyl alcohol as the solvent of the melt-adhesive coatings. However, benzyl alcohol is expensive and accordingly is not practicable.
Another method is to decrease the amount of a solvent to be used in the melt-adhesive coatings so as to increase the concentration of a resin. In this case, the residual solvent problem would be solved, but the wire preparation becomes difficult since the viscosity of the melt-adhesive coatings is high at usual coating temperature. Therefore, in case of using melt-adhesive coatings having a high resin concentration, the coatings must be applied at an elevated temperature to obtain a viscosity suitable for the application. However, in this case, the solvent evaporates from the melt-adhesive coatings during the application and, therefore, not only the working environment is deteriorated, but also the viscosity increases with the lapse of time to finally make the application operation difficult.
In order to solve the problem encountered when using melt-adhesive coatings having a high resin concentration, it is proposed to decrease the molecular weight (relative viscosity) of a copolyamide resin to be used in the coatings. Since the viscosity of the coatings can be lowered by this method, it is possible to achieve a high resin concentration. However, coils molded from a self-bonding insulated wire having a bonding layer of a low molecular weight copolyamide or a litz wire made thereof have the problems not only that the initial strain is large, the heat distortion resistance at ordinary and high temperatures is inferior and the adhesive strength is low, but also that when the coils are used in deflecting yoke or the like, the adhesive strength between the wires is lowered by a heat generated at the time of use. The coils having a lowered adhesive strength are easy to become loose, resulting in a cause of misconvergence of displayed image.
As a means to solve such a problem, for example, JP-A-5-59329 discloses that a tough film is obtained by preparing a coating solution containing as a main component a polyamide having a terminal carboxyl group and a terminal amino group in a ratio of 1:4 to 4:1 by mole and having a relative viscosity of not more than 1.8 measured at 25xc2x0 C. with respect to a 0.5% by weight cresol solution thereof, applying the coating solution onto a substrate and thermally polymerize the polyamide during or after drying the applied solution. It is disclosed that the concentration of the coating solution is from 1 to 20% by weight. However, the concentration of the coating solutions used in the working examples is 10% by weight. Decrease of the residual solvent in the formed bonding layer is not expectable at this concentration. Also, since the polyamide having a relatively low molecular weight is subjected to thermal polymerization, the initial strain becomes large.
Also, JP-A-5-225831 discloses a self-bonding insulated wire which is shape-retainable at high temperatures and which has a good fluidity and is easy to melt-bond to each other. In working examples thereof, a melt-bonding film composed mainly of a copolyamide having a melting point of 110xc2x0 C. and a reduced specific viscosity of 1.02 to 1.30 is formed on a conductor through an interposed insulated film. A cresol solution of the copolyamide is used as a melt-adhesive coating for forming the melt-bonding film. The resin concentration of the coating is disclosed to be 200% by weight, but 200% appears to be clerical error. Since the resin concentration is unknown, it is not clear whether JP-A-5-225831 achieves the decrease of the residual solvent in the formed bonding layer. The bonding layer of self-bonding insulated wire according to JP-A-5-225831 has no sufficient strength, and also the initial strain appears to be large.
Further, it is conceivable that the amount of residual solvent may be decreased by elevating the oven temperature for baking the applied coating. However, in that case, thermal decomposition of a melt-adhesive resin is easy to occur and, therefore, the heat distortion resistance of the obtained deflecting yoke coil is lowered and the coil is easy to become inoperable. No proposal to solve the problem encountered when elevating the baking temperature has been made.
It is an object of the present invention to solve the problems as mentioned above on self-bonding insulated wires and self-bonding litz wires.
A further object of the present invention is to provide a self-bonding insulated wire which has excellent heat distortion resistance and adhesive strength and a decreased amount of a residual solvent and which can provide coils having a small initial strain and a small dimensional change with good workability and productivity.
A still further object of the present invention is to provide a self-bonding litz wire comprising the self-bonding insulated wire having such advantages.
These and other objects of the present invention will become apparent from the description hereinafter.
It has been found that the resin concentration of a coating used for forming the bonding layer of self-bonding insulated wires can be increased without deteriorating the coating workability by using a specific crystalline copolyamide as the melt-bonding component of the bonding layer.
Thus, in accordance with the present invention, there is provided a self-bonding insulated wire having a bonding layer on an insulated wire, wherein said bonding layer is formed by applying and baking a coating comprising a crystalline copolyamide having a melting point within the range of 105 to 150xc2x0 C. and a relative viscosity of 1.4 to 1.6 measured at 25xc2x0 C. with respect to a 0.5% by weight m-cresol solution thereof, and the relative viscosity of a 0.5% by weight solution of said formed bonding layer dissolved in m-cresol is from 1.4 to 1.6 at 25xc2x0 C.
In the present invention, a coating containing a specific crystalline copolyamide is used for forming the melt-bonding layer of self-bonding insulated wires, whereby a higher resin concentration can be achieved as compared with conventional coatings for forming the melt-bonding layer without lowering the coating workability. Accordingly, the residual solvent in the self-bonding insulated wire can be remarkably decreased to diminish the problems on odor and working environment. Also, the productivity is improved. Moreover, the initial strain is small and the heat resistance is excellent.
It has also been found that the initial strain can be decreased by using the crystalline copolyamide in combination with a high-melting nylon, and that the thermal deterioration of every polyamide resins incorporated in the bonding layer can be prevented by incorporating an antioxidant into the bonding layer, whereby it is possible to provide deflecting yoke coils that the wire-wire adhesive strength is hard to lower during the use.
Thus, the present invention also provides a self-bonding insulated wire having a bonding layer on an insulated wire, said bonding layer comprising a crystalline copolyamide having a melting point within the range of 105 to 150xc2x0 C. and a relative viscosity of 1.4 to 1.6 measured at 25xc2x0 C. with respect to a 0.5% by weight m-cresol solution thereof, and at least one member selected from the group consisting of a high-melting nylon having a melting point within the range of 200 to 300xc2x0 C. and an antioxidant.
From these self-bonding insulated wires of the present invention, self-bonding litz wires are prepared by stranding a plurality of the wires in a known manner.